The present invention relates to a method for converting a tone of a picture made use in plate-making process for producing reproduced pictures such as printed pictures, various pictorial hard copies of high quality. This invention is, more specifically, concerned with a novel tonal conversion method for a picture employed when a reproduced picture of a half tone is produced from a color photographic picture of a continuous tone as an original picture.
It is a matter of course that when a color photographic picture in a continuous tone is used as an original picture to make a reproduced picture such as a printed picture by using a color scanner as a plate-making device or to output a pictorial hard copy of a high quality by using a scanner or a high-performance printer, the continuous tone of the original picture should be converted into a halftone expressing a reproduced picture.
A term "tonal conversion of a picture" mentioned above means that density information of each pixel of an original picture is converted into density information of a reproduced picture system.
There have been known in a field of production of a reproduced picture various reproducing techniques (i.e., density representing techniques) to reproduce the density information of an original on a reproduced picture, in any one of which the tone of the original picture is converted.
The tonal conversion closely relates to conversion of density information of pixels along with conversion of color tone of pixels. The detail of this will be stated later.
Typical techniques employed currently to represent density of a reproduced picture are as follows:
(a) Typically used to produce printed pictures. A covering rate of a given pixel is changed according to a size of a dot (as called "dot area % value"). Namely, a covering rate of a given pixel is changed by varying a size of a dot to reproduce the density information of an original picture on a printed picture. PA1 (b) There is another technique where a covering rate of a given pixel is changed by changing an arrangement of dots of the same size, as contrasted with the above multiple valued dot area technique where a size of a dot is changed. This technique is called "binary valued dot area tonal conversion representation", applied in the same field such as thermally molten transfer printers, ink jet printers, and the like. PA1 (c) There is still another technique where the density of a pixel of a given size is changed. This is based on the same principle as the reproducing system of a continuous density seen in general photographic materials. This technique is referred "a direct density tonal representation", applied in sublimation type thermal transfer printers (sublimation type printers) and the like. PA1 converting the picture information values (Xn) correlating with light intensity into tonal intensity values (y) conducting a tonal conversion formula: EQU y=y.sub.H +[.alpha.(1-10.sup.-k.multidot.x)/(.alpha.-.beta.)].multidot.(y.sub.s -y.sub.H)
This technique is referred "a multiple valued dot area tonal representation", applied to ink jet printers, thermally molten transfer printers (thermal printers), and the like, in addition to a printed picture producing device.
As stated above, various reproducing techniques are used to produce reproduced pictures such as printed pictures, hard copies outputted from a high-performance printers and the like from original pictures of a continuous tone. Incidentally, a term "reproduced picture" referred in this invention should be interpreted in the broadest sense including printed pictures and outputted hard copies of a high-performance printer. It is a material subject in such techniques to faithfully reproduce, gradation and color tone of an original picture in a reproduced picture, with operational regularlity.
It is, however, the present condition that the subject relating to reproducibility, mentioned above, cannot be rationally overcome in spite of recent remarkable improvements in the filed of picture reproduction.
The main cause of the present condition lies in that, in a technique to faithfully reproduce a tone (including gradation and color tone) of an original picture on a reproduced picture or in a technique to control (correct or alternate) the tone of an original picture, a tone non-linearly converting and processing technique (i.e., a tonal conversion technique of a picture or a tonal conversion method of a picture) applied relating to a density area of a picture lacks theory, thus being non-scientific and irrational, although the technique must be basic.
This point will be next described in detail by way of description of a processing technique of a printed picture, which is a typical example in a field of production of reproducing pictures.
Inventors of the present invention have an opinion that knowledge accumulated in the field of plate-making and printing have not been taken full advantages of in current plate-making systems and equipments highly developed such as monochrome scanners, color scanners and the like, a technique to design such highly developed equipment has been founded on the color-separation theory based on a photographic masking method that is non-systematic, and various improvements often made on the systems and equipments have been merely a symptomatic treatment based on the color-separation theory.
In the conventional design technique of a scanner system, color compensation and color correction (color masking) are considered to be primary and tone adjustment of a picture is considered to be secondary when a photographic original in a continuous tone is converted into a printed reproduced picture in a halftone through a color-separation work. As a result, the present technology fails to accomplish high stability and high reproducibility of picture quality when producing a printed pictures.
As stated, there has not been found any appropriate measure to correlate a density value of a given sample point (i.e., a pixel point) in a picture of a continuous tone to a dot area % value of a dot corresponding to the given sample point on a reproduced picture of a halftone, using the conventional tonal conversion technique used to convert a picture of a continuous tone into a picture of a halftone.
From a standpoint that a technique enabling rational tonal conversion relating to density of each pixel of a picture should be considered to be primary rather than an improvement of two important technical elements, that is, a gradation control technique and a color correction technique, in order to rationally produce a printed picture, the inventors of this invention have been making an aggressive research to do away with a limitation lying in the non-rational, non-scientific conventional tonal conversion techniques.
As a result of the research, the inventors have proposed a novel tonal conversion method in which a basic density value of a given sample point on a picture of a continuous tone is correlated with a dot area % value of a point corresponding to the given sample point on a reproduced picture of a halftone. It has been found that the novel tonal conversion method that is rational and scientific is not only applicable to the conventional plate-making systems and equipments, but also is able to get rid of the limitation lying in the conventional color-separation techniques and to exhibit an excellent effect. The details of the preceding tonal conversion method having been proposed by the inventors are disclosed in specifications of Japanese Patent Laid-Open Publication No. HEI 3-3477 of Patent Application No. HEI 1-135825 (corresponding to U.S. Patent application Ser. No. 07/481,054 and the following U.S. Patent Application) and Japanese Patent Application No. HEI 2-55204 (corresponding to U.S. Patent No. 5,072,305).
The tonal conversion methods which has been proposed by the inventors employ a specific tonal conversion formula analogous with a tonal conversion formula according to the present invention, when used upon converting a tone of a picture, (but, the operational conditions therebetween are quite different from each other, as will be described later.) The preceding tonal conversion methods admit of improvement in order to produce with higher reproducibility a printed picture of a superior quality.
The preceding tonal conversion methods are considered to be insufficient in some points that a constant in the preceding tonal conversion formula needs to be determined from density values measured in a highlight area (H) and a shadow area (S) in order to operate the tonal conversion formula, besides the density values in H and S area are measured unavoidably with variations, more or less. To overcome the above problem is necessary to produce a printed picture of an excellent quality with high reproducibility, as desired.
From the above, it will be appreciated that the problem lying in the preceding tonal conversion method of a picture by the inventors exists commonly in a field of production of general reproduced pictures, although the above description has been made by way of the field of production of printed pictures.
An object of the present invention is therefore to provide a novel tonal conversion method of a picture, applicable to a field of production of various reproduced pictures.
More specifically, it is common that picture information (i.e., density information) is obtained from an original picture and the obtained picture information is converted by means of a tone converting part (i.e., a tone converter) in a reproduced picture producing system. The tonal conversion method of this invention is intended to be applied in the above process so as to carry out the tone conversion work rationally.
In particular, according to the present invention, an effect due to variations unavoidably occurring in density values measured in the H and S areas designated respectively on a color photographic original picture may be recognizably reduced. The density information free from such variations is used to produced a reproduced picture of a superior quality with high reproducibility as desired by operating a specific tonal conversion formula proposed in this invention.